In recent years, there has been development of organic semiconducting materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, photovoltaic (PV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
Pentacene has shown promise as an organic semiconducting material. Pentacene has been described as requiring a highly crystalline structure in order to provide a molecular orientation which results in good charge mobility. Thus, in the prior art, thin films of pentacene have been vapour deposited, due in part to the fact that pentacene is rather insoluble in common solvents. However, vapour deposition requires expensive and sophisticated equipment. In view of the latter problem, one approach has been to apply a solution containing a precursor pentacene and then chemically converting, for example by heat, the precursor compound into pentacene. However, the latter method is also complex and it is difficult to control in order to obtain the necessary ordered structure for good charge mobility.
Soluble pentacene compounds comprising silylethynyl groups, like 6,13-bis(triethylsilylethynyl)pentacene have recently been described in the prior art as organic semiconducting compounds [1,2]. This compound exhibits high performance as the semiconducting layer in an organic field-effect transistor (OFET), with mobility of 0.4 cm2/Vs and current on/off ratio of 106 measured [3]. Meanwhile, significant work has been undertaken by various groups to design and prepare soluble pentacene materials that offer even higher performance in terms of semiconducting properties, and that also show enhanced processability and environmental stability.
However, the properties of the bis(silylethynyl)pentacenes described in prior art still leave room for further improvement. For example, pentacene type molecules degrade in the presence of air and light due to a photooxidation process [21,22].
One aim of the present invention was to provide further pentacene compounds that are useful as organic semiconducting materials.
In prior art 6,13-bis(trialkylsilylethynyl)pentacenes with additional substituents in 1-, 2-, 3-, 8-, 9-, 10- and/or 11-position are disclosed [23].
By adding substituents in these positions, which are prone to the above-described photo-oxidation process, it is possible to hinder the degradation. This leads to polyacenes that are useful as charge transport and semiconducting materials and have improved solubility and charge carrier mobility and improved stability especially against air, heat and light. Furthermore, when these substituted polyacenes are provided in a formulation together with an organic binder, improved semiconducting materials with good processibility are obtained which do still show a surprisingly high charge carrier mobility.
The inventors of the present invention have now found that especially pentacenes with substituents in 1-, 4-, 8- and 11-position do unexpectedly show high charge carrier mobility, good solubility in standard organic solvents, and good processibility.
However, it was also found that such substituted pentacenes are difficult to synthesize. Generally, pentacene ring-network precursors have previously been constructed using either the Aldol condensation [4] or the Cava reaction [5]. However, the inventors have found that for 1,4,8,11-tetrasubstituted 6,13-bis(triethylsilylethynyl) pentacene both the Aldol and Cava methodologies yield little success in constructing the pentacene ring-network precursors in high and reproducible yields. Therefore a more successful alternative method is high desirable.
Thus, another aim of the present invention was to find an improved synthesis method for 1,4,8,11-substituted pentacenes. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It was now found that these aims can be achieved by providing methods and materials as claimed in the present invention. In particular, this invention relates to a new synthetic route to prepare 1,4,8,11-tetrasubstituted pentacene, which circumvents the key drawbacks of the previous routes based around the Aldol condensation or the Cava reaction. Furthermore, it provides novel 1,4,8,11-tetrasubstituted pentacenes with improved properties, especially high charge carrier mobility, high solubility and good processibility.